(1) Field of the Invention
The present invention relates to drive units for legged robots and control method thereof. The invented drive unit powers the robot's joint by using flexible members, and the invented control method provides passive, passive-dynamic and active walking modes and transition between the modes, without using any additional physical means.
(2) Description of Related Art
In general, two main modes of walking for biped, quadruped, or other legged walking robots are known. One is passive walking and the other is active walking (for example, refer to T. McGeer, “Passive dynamic walking,” Int. J. Robot. Res., Vol. 9, No. 2, pp. 62-82, April 1990). Characteristically for passive walking is that the joints of robot's legs are not powered and the robot walks passively by descending a shallow slope. This type of walking resembles what is observable in the nature, and enables walking without energy supply. Mechanical transport cost factor, which is defined as a ratio of consumed energy to the robot's weight and traveled distance is known to be less than 0.1 for passive walking (refer to S. Collins, A. Ruina, R. Tedrake, M. Wisse, “Efficient Bipedal Robots Based on Passive Dynamic Walkers,” Science, Vol. 307, pp. 1082-1085, February 2005, page 1083). However, due to lack of energy supply, a sustained passive walk on a horizontal plane is impossible, additionally, due to lack of active control, the walking on an uneven surface is also impossible.
Majority of recently developed legged robots walk in a way that is described as active walking (for example U.S. Pat. No. 7,278,501, U.S. Pat. No. 7,441,614). In the disclosed patented robots, the robot's joints and the driving motors are continuously coupled and the joints are powered and controlled throughout the whole walking step cycle. The result of active walking is higher energy consumption and the total energy cost defined as a ratio of consumed energy to the robot's weight and traveled distance is 3 or higher (refer to S. Collins, A. Ruina, R. Tedrake, M. Wisse, “Efficient Bipedal Robots Based on Passive Dynamic Walkers,” Science, Vol. 307, pp. 1082-1085, February 2005, page 1083), but the robots are more stable and robust when walking on an uneven terrain.
In recently published research results (Carnegie Mellon University, MIT, Delft University) it is shown that the passive walking, which is originally possible only on descending slopes, can be realized also on horizontal planes, if the needed energy is supplied to the joints to sustain the walking Such walking is called-passive dynamic walking (S. Collins, A. Ruina, M. Wisse, “A Three Dimensional Passive-Dynamic Walking Robot With Two Legs and Knees,” Int. J. Robotics Research, Vol. 20, No. 7, pp. 607-615, July 2001).
An example of passive-dynamic walking is presented in M. Wisse, D. G. E. Hobbelen, A. L. Schwab, “Adding an Upper Body to Passive Dynamic Walking Robots by Means of a Bisecting Hip Mechanism,” IEEE Trans. Rob., Vol. 23, No. 1, pp. 112-123, February 2007, where knees are equipped with locking mechanism, ankles are fixed, and toes have circular sole shape. Sustained walking on a horizontal plane is realized in such a way that at the instant when a support leg becomes a swing leg, the knee of the swing leg is unlocked and the hip is for a short duration powered with a torque, so that the swing leg kicks forward. A drawback of the proposed method is that the free motion of a swing leg cannot be achieved with a servomotor and gears unless a designated clutch mechanism to separate the joint from the drive unit is used. Without a clutch mechanism the motor and the joint are continually coupled and the motor must actively follow the motion of a leg, which increases the total energy consumption.
Muscle type actuators are beneficial in the sense that they allow free motion of a swing leg, therefore, McKibben actuators are used in M. Wisse, D. G. E. Hobbelen, A. L. Schwab, “Adding an Upper Body to Passive Dynamic Walking Robots by Means of a Bisecting Hip Mechanism,” IEEE Trans. Rob., vol. 23, no. 1, pp. 112-123, Feb. 2007, p. 118, FIG. 9. By using a pneumatic artificial muscle, a swing leg can freely move when muscles are not pressurized, but the control of a pneumatic muscle itself is highly nonlinear and suffers from a dead time in the response, therefore, stable walking and good performance is difficult to achieve. Another disadvantage of the pneumatic muscles and other artificial muscles is that they require energy source in a form of compressed gas, heat, or chemical energy, which are difficult to provide, store, and control, and therefore are in total the cause for poor performance of a robot.